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The performance of composite baseball bats typically improves with age. The reason for this is the breaking-in process; in other words, the force of striking the ball over and over eventually breaks down the bat's composite fibers and resinous glues that different manufacturers use. This improves the bat's Bat Performance Factor (BPF) or Ball Exit Speed Ratio (BESR). These factors are directly related to the rate of speed at which a struck ball comes off the bat; with higher speeds representing a greater danger to players in the infield (closer to the batter). Bats used in Little League play are required to remain below a league-specified values to be allowed in league play when new and after Accelerated Break-In (ABI).
One method of accelerating this is called Bat rolling. This is a process in which a machine uses rollers to apply pressure perpendicular to the composite baseball bat's surface, breaking down the composite fibers, etc.. This causes the fibers to expand and create a trampoline effect, thus projecting the ball further and faster than possible with an aluminum bat. As mentioned, bats eventually break-in on their own; rolling a bat simply speeds up the breaking-in process.
The process of bat rolling began in Southern California, around late 1999 to early 2000, but grew in worldwide popularity significantly after 2002. A 17-year-old with an English wheel machine called a "Chepe" originated the process. Eventually improvements were made to the machinery by English wheel manufacturers. Modern machines have bigger nylon rollers to roll the bat either parallel or perpendicular, without causing much damage to the barrel.